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Critical Radius of Insulation Calculator — Cylinder & Sphere
Calculate the critical insulation radius for cylinders or spheres from insulation conductivity and external convection coefficient.
Last updated: September 17, 2026
Results
Conductivity and h must be positive
Formula, example and assumptions
Cylinder rc=k/h; sphere rc=2k/h.
With k=0.05 W/mK and h=10 W/m²K, cylinder rc=5 mm and sphere rc=10 mm.
- External convection coefficient is treated as constant.
- The critical-radius concept concerns the competition between conduction resistance and growing external area.
Frequently asked questions
What does the Critical Radius of Insulation Calculator calculate?
Calculate the critical insulation radius for cylinders or spheres from insulation conductivity and external convection coefficient.
Which units should I use?
The input labels show the required SI units. Keep properties and geometry consistent before using the result in another engineering calculation.
Can I use this result for design?
Use it for transparent engineering estimates and checks. Validate property data, geometry, correlations and applicable codes for safety-critical design.